This invention relates to a discontinuous process for preparing (1,4-cyclohexanedimethylene) terephthalate polymers starting from terephthalic acid.
(1,4-Cyclohexanedimethylene) terephthalate polymers and copolymers have long been known and are characterized by their high melting points (U.S. Pat. Nos. 2,901,466, 3,261,812, 4,042,570).
Although the use of terephthalic acid as a raw material is not ruled out, only examples starting with dialkyl terephthalates are given in the literature. The high temperature required to assure the melt phase during polycondensation lead to decomposition products, which also act as chain terminators and thus prevent the formation of high-molecular polymers (U.S. Pat. No. 3,271,370). In order to counteract this degradation, it has been proposed that diol and dialkyl terephthalate be used in approximately equimolar amounts (U.S. Pat. No. 3,271,370), and that zinc fluorotitanate be used as an especially active catalyst (U.S. Pat. No. 3,068,204), that the ester exchange step be performed under nitrogen (U.S. Pat. No. 4,042,570) or that a stabilizing comonomer such as gentisic acid derivatives be polymerized into the product (German Patent Application 1,495,570).
This tendency to form decomposition products which then function as chain terminators occurs to a significantly greater extent when terephthalic acid is used as a raw material, where the aforementioned countermeasures have proven to have little or no effect.
In attempting to apply the examples in the literature for dimethyl terephthalate to terephthalic acid, it has been found that after a polycondensation time of 40 to 60 minutes at 295.degree. to 305.degree. C. the degradation reaction greatly exceeds the condensation reaction and is accompanied by an exponential decline in the intrinsic viscosity plus discoloration of the polymer. After prolonged thermal stress, decomposition down the oligomer level has even been observed. The statement made in U.S. Pat. No. 3,271,370 that an excess of diol is a disadvantage has not been confirmed when using terephthalic acid.
These chain terminating substances are metabolites of 1,4-bis(hydroxymethyl)cyclohexane that are formed by dehydration of the free or esterified CH.sub.2 OH groups attached to the cyclohexane ring, thus forming methylenecyclohexane and methylcyclohexene derivatives. This dehydration is catalyzed by acids in general and by terephthalic acid in particular and by the COOH end groups formed by cleavage of ester linkages during the polymerization process.